(1) Field of the Invention
The present invention relates to a drying method. More particularly, the present invention relates to a method for preparing a dry product from a food, a medicine or the like without substantial deterioration of nonvolatile components contained therein. Furthermore, the present invention relates to a method for drying a food, a medicine or the like without discharge of volatile components contained in the substance to be dried, in the form of a gas to the outer atmosphere.
(2) Description of the Prior Art
As the method for drying a liquid food or medicine at a relatively low temperature to form a powdery dry product, there have been known a so-called freeze-drying method and a hot air drying method.
In the freeze-drying method, a solution or the like is kept under a high vacuum and ice formed by freezing is sublimated to enhance drying. According to this method, however, since the structure of cells in the substance to be dried is broken, it is often observed that the resulting dry powdery product is different from the original liquid food or the like with respect to the composition of the ingredients and the texture. Furthermore, in order to increase the drying speed, such a high vacuum as about 10.sup.-2 mmHg is necessary and hence, the method requires expensive equipment. Moreover, heat necessary for sublimation of ice can only be supplied by heat conduction, and as dried zone of a food or the like advances from the bottom portion in contact with a heating wall to the surface, the food or the like becomes porous and its heat conductivity is reduced. Accordingly, the time necessary for drying is prolonged because of reduction of the heat conductivity and resulting reduction of the drying speed, and therefore a large amount of energy is required for completion of the drying. Still further, according to this method, although water separated in the form of ice can be sublimated and removed, water contained in solids cannot be removed completely and it is impossible to reduce the water content below several percent, because some water remains adsorbed in solids.
In the hot air drying method, by contacting a solution or the like with hot air, its water and the like are removed in the state of vapour by the drying gas. This method is advantageous in that the operation is relatively simple, but a large quantity of hot air as the drying gas is necessary and deterioration of the substance to be dried is often caused by the high temperature operation.
Moreover, these conventional drying methods involve a problem of air pollution by odour and, smelling components and waste of valuable flavor components and other volatile components discharged together with water vapor and drying air.